| Abstract |
In an active rift like that of Ethiopia, rifting is preceded by volcanism and doming. These are effects of the deep-rooted mantle up welling or ìplumeî. The process is continuous even in the developed rift, particularly in its axial part. This situation has a strong effect on the stratification of the rocks, situated on both sides of the rift axis. That is, due to the up doming at the axial part, the rocks are remarkably inclined outwards of the axis in both directions. The rift margins being the highest picks, the general slope of the whole dome is outwards of the rift, hence it controls the regional drainage and the ground water flow. The strong inclinations of rock beds can be easily observed on global scale topographic maps, images and is clearly indicated by the regional distribution of the basement and volcanic rocks of Ethiopia. This is again reflected on the drainage pattern i.e., the most important rivers in the Northwestern Plateau flow to the northwest and those on the Southeastern Plateau flow towards southeast, in both directions away from the rift. Therefore, water circulation both on the surface and in subsurface at the axial parts of the rift is minimal. The axial part of a rift is hotter than the marginal and external parts. To have a geothermal system, there needs to be a heat source and a good recharge together with an aquifer and a cap rock. These conditions could be fulfilled, except for the recharge, in the axial part of a rift. A second limitation for a geothermal system in a Continental Rift Zone is the block forming nature of the fractures or faults that allows fluid circulation only along the weak zones. Deep geothermal exploration wells in the Main Ethiopian rift (Aluto-Langano) and Afar (Tendaho) geothermal fields have proven the existence of high temperature (sometimes more than 350?C), but relatively small amounts of steam production. |